Synthesis of Nucleoside-like Molecules from a Pyrolysis Product of Cellulose and Their Computational Prediction as Potential SARS-CoV-2 RNA-Dependent RNA Polymerase Inhibitors

(1R,5S)-1-Hydroxy-3,6-dioxa-bicyclo[3.2.1]octan-2-one, available by an efficient catalytic pyrolysis of cellulose, has been applied as a chiral building block in the synthesis of seven new nucleoside analogues, with structural modifications on the nucleobase moiety and on the carboxyl- derived unit. The inverted configuration by Mitsunobu reaction used in their synthesis was verified by 2D-NOESY correlations, supported by the optimized structure employing the DFT methods. An in silico screening of these compounds as inhibitors of SARS-CoV-2 RNA-dependent RNA polymerase has been carried out in comparison with both remdesivir, a mono-phosphoramidate prodrug recently approved for COVID-19 treatment, and its ribonucleoside metabolite GS-441524. Drug-likeness prediction and data by docking calculation indicated compound 6 [=(3S,5S)-methyl 5-(hydroxymethyl)-3-(6-(4-methylpiperazin-1-yl)-9H-purin-9-yl)tetrahydrofuran-3-carboxylate] as the best candidate. Furthermore, molecular dynamics simulation showed a stable interaction of structure 6 in RNA-dependent RNA polymerase (RdRp) complex and a lower average atomic fluctuation than GS-441524, suggesting a well accommodation in the RdRp binding pocket.


Introduction
Cellulose is the most biosynthesized organic substance on earth and is an abundant component of plants. According to the biorefinery concept, it represents a key biomass to produce fuels and chemicals. In line with the sustainability of bioresources, the conversion of the carbohydrate content in lignocellulosic waste represents an interesting valorization. Biomass-derived compounds are often rich in functionalization and stereocenters, so that to be crucial chiral pools for the synthesis of non-racemic molecules.
The bio-oil produced by catalytic pyrolysis of cellulose contains anhydromonosaccharides and among them the hydroxylactone LAC (1) (=(1R,5S)-1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one) [1] in Figure 1 turned out to be a compound with added value as chiral building block to be used in asymmetric synthesis. In particular, an improved production on the gram scale by pyrolysis in the presence of cheap and eco-friendly catalyst [2] has favored the use of LAC to obtain enantiomerically pure bioactive molecules. This was used to produce the new branched δ-sugar amino acid 2 (Figure 1) acting as a conformationally restricted isostere of the glycine-alanine dipeptide with potential applications in peptidomimetics [3]. Furthermore, the structural similarity of this amino acid with L(+)-muscarine inspired us the production of a series of new compounds showing affinity for human cloned muscarinic receptors [4]. In the present work LAC is used to obtain nucleoside-like molecules with potential biological activities. Nucleosides consist of a nucleobase, typically a purine or pyrimidine, and a five-carbon sugar, displaying a remarkable chemical diversity in nucleoside-based secondary metabolites. Both natural and synthetic nucleosides and nucleotides ( Figure 2) exhibit peculiar biological properties. Nucleosides or their analogues are used in the treatment of cancer and viral infections. Regarding their antiviral application, over 25 nucleoside and nucleotide analogues were approved as therapeutic agents [5]. The majority of them target enzymes involved in virus replication, including RNA viral polymerases which have proven to be valid targets for the development of antiviral agents, because all RNA viruses encode an RNA-dependent RNA polymerase (RdRp). Nucleotide and nucleoside inhibitors are usually administered as prodrugs, which are metabolized to their active triphosphate once inside the cell [6]. In the last few years, the disease causing the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), named COVID-19, has spread globally, causing a public health emergency. Vaccines aim to prevent infection and a series of vaccines have been made available to fight this pandemic relatively quickly. However, there is also the pressing need to develop additional effective therapy. The use of antibodies is proving to be a valid treatment in some cases and antiviral drugs can offer an effective remedy to treat the worst symptoms, especially for immunosuppressed people. Potential repurposed antiviral drugs are currently under evaluation, including agents active against HIV, Ebola and Zika infections [7].
Remdesivir (GS-5734, Figure 3) is active against a series of viruses, by inhibiting RdRp to stop viral replication [8]. Recently, it has been approved to treat COVID-19 by the United States Food and Drug Administration (FDA). Acting as a monophosphate prodrug, remdesivir is the precursor of the 10-cyano adenosine nucleoside GS-441524 ( Figure 3), because after administering it is subjected to an in vivo bioactivation, producing GS-441524 as the predominant metabolite circulating in the bloodstream [9]. Displaying a higher efficacy than remdesivir and several advantages [9], it has also been authorized for COVID-19 treatment. In the present work LAC is used to obtain nucleoside-like molecules with potential biological activities. Nucleosides consist of a nucleobase, typically a purine or pyrimidine, and a five-carbon sugar, displaying a remarkable chemical diversity in nucleoside-based secondary metabolites. Both natural and synthetic nucleosides and nucleotides ( Figure 2) exhibit peculiar biological properties. Nucleosides or their analogues are used in the treatment of cancer and viral infections. Regarding their antiviral application, over 25 nucleoside and nucleotide analogues were approved as therapeutic agents [5]. The majority of them target enzymes involved in virus replication, including RNA viral polymerases which have proven to be valid targets for the development of antiviral agents, because all RNA viruses encode an RNA-dependent RNA polymerase (RdRp). Nucleotide and nucleoside inhibitors are usually administered as prodrugs, which are metabolized to their active triphosphate once inside the cell [6]. In the present work LAC is used to obtain nucleoside-like molecules with potential biological activities. Nucleosides consist of a nucleobase, typically a purine or pyrimidine, and a five-carbon sugar, displaying a remarkable chemical diversity in nucleoside-based secondary metabolites. Both natural and synthetic nucleosides and nucleotides ( Figure 2) exhibit peculiar biological properties. Nucleosides or their analogues are used in the treatment of cancer and viral infections. Regarding their antiviral application, over 25 nucleoside and nucleotide analogues were approved as therapeutic agents [5]. The majority of them target enzymes involved in virus replication, including RNA viral polymerases which have proven to be valid targets for the development of antiviral agents, because all RNA viruses encode an RNA-dependent RNA polymerase (RdRp). Nucleotide and nucleoside inhibitors are usually administered as prodrugs, which are metabolized to their active triphosphate once inside the cell [6].  In the last few years, the disease causing the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), named COVID-19, has spread globally, causing a public health emergency. Vaccines aim to prevent infection and a series of vaccines have been made available to fight this pandemic relatively quickly. However, there is also the pressing need to develop additional effective therapy. The use of antibodies is proving to be a valid treatment in some cases and antiviral drugs can offer an effective remedy to treat the worst symptoms, especially for immunosuppressed people. Potential repurposed antiviral drugs are currently under evaluation, including agents active against HIV, Ebola and Zika infections [7].
Remdesivir (GS-5734, Figure 3) is active against a series of viruses, by inhibiting RdRp to stop viral replication [8]. Recently, it has been approved to treat COVID-19 by the United States Food and Drug Administration (FDA). Acting as a monophosphate prodrug, remdesivir is the precursor of the 10-cyano adenosine nucleoside GS-441524 ( Figure 3), because after administering it is subjected to an in vivo bioactivation, producing GS-441524 as the predominant metabolite circulating in the bloodstream [9]. Displaying a higher efficacy than remdesivir and several advantages [9], it has also been authorized for COVID-19 treatment. In the last few years, the disease causing the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), named COVID-19, has spread globally, causing a public health emergency. Vaccines aim to prevent infection and a series of vaccines have been made available to fight this pandemic relatively quickly. However, there is also the pressing need to develop additional effective therapy. The use of antibodies is proving to be a valid treatment in some cases and antiviral drugs can offer an effective remedy to treat the worst symptoms, especially for immunosuppressed people. Potential repurposed antiviral drugs are currently under evaluation, including agents active against HIV, Ebola and Zika infections [7].
Remdesivir (GS-5734, Figure 3) is active against a series of viruses, by inhibiting RdRp to stop viral replication [8]. Recently, it has been approved to treat COVID-19 by the United States Food and Drug Administration (FDA). Acting as a monophosphate prodrug, remdesivir is the precursor of the 10-cyano adenosine nucleoside GS-441524 ( Figure 3), because after administering it is subjected to an in vivo bioactivation, producing GS-441524 as the predominant metabolite circulating in the bloodstream [9]. Displaying a higher efficacy than remdesivir and several advantages [9], it has also been authorized for COVID-19 treatment.  We report here on the synthesis of a series of new nucleoside-like molecules starting from the chiral building block LAC available from an efficient ecofriendly catalytic pyrolysis of cellulose. The drug-likeness properties of these products have been evaluated obtaining a favorable pharmacokinetic profile and the related phosphate compounds have been studied in silico as inhibitors of SARS-CoV-2 RNA-dependent RNA polymerase providing promising results when compared with remdesivir and its metabolite.

Synthesis of Nucleosides 5-11
Seven nucleoside analogues were synthesized from the same common precursor 3, available by protecting the primary alcohol as tertbutyldimethylsilyloxy derivative of the methyl ester 2 accessible from the lactone LAC (1, Scheme 1). Structural modifications are introduced on the nucleobase moiety (  We report here on the synthesis of a series of new nucleoside-like molecules starting from the chiral building block LAC available from an efficient ecofriendly catalytic pyrolysis of cellulose. The drug-likeness properties of these products have been evaluated obtaining a favorable pharmacokinetic profile and the related phosphate compounds have been studied in silico as inhibitors of SARS-CoV-2 RNA-dependent RNA polymerase providing promising results when compared with remdesivir and its metabolite.

Synthesis of Nucleosides 5-11
Seven nucleoside analogues were synthesized from the same common precursor 3, available by protecting the primary alcohol as tertbutyldimethylsilyloxy derivative of the methyl ester 2 accessible from the lactone LAC (1, Scheme 1). Structural modifications are introduced on the nucleobase moiety ( Figure 2) (as chloropurine in 5, 8, adenine in 9-11, methylpiperazinyl purine in 6, methylaminopurine in 7), and on the carboxyl-derived unit (methyl ester in 5-7 and 9, i-propyl ester in 8 and 10 and N-methylamide in 11).
In detail, compound 4 was obtained by a Mitsunobu reaction of 3 with 6-chloropurine in THF using diethyl azodicarboxylate (DEAD) and triphenylphosphine [10]. By treating 4 with tetrabutylamoniun fluoride, the compound 5 was produced. Based on Mitsunobu mechanism, a good leaving group is generated by reaction of the alcohol with triphenylphospine followed by a bimolecular nucleophilic substitution providing inversion of stereochemistry. 2D-NOESY experiment acquired for compound 5 in CDCl 3 confirmed the derived (S) configuration at C-2, starting from the (R)-assigned in LAC. A correlation was observed between proton at 4.39 ppm assigned at C-4 position (of known (S) configuration based on LAC as starting chiral building block) with H-3 at 3.11 ppm, and for the other proton in position 3 at 2.76 ppm with the singlet at 8.40 ppm assigned to H-2 in purine unit. Evidence from these experiments found support in the energy-minimized structure of 5 by density functional theory (DFT) calculation in chloroform, where the distances (2.307 Å and 2.033 Å, respectively) are compatible with the observed NOE ( Figure 4). The N-methylpiperazinyl moiety in analogue 6 and the N-methyl unit in derivative 7 were efficiently provided in good yields by treating the chloropurinyl compound 5 with the corresponding amines under microwave irradiation (M W ) in methanol. Similar technique was adopted to obtain analogue 8 by triazabicyclodecene (TBD)-catalyzed transesterification of the methyl ester 5 with i-propanol. As derivatives from 5, the absolute configurations of the analogues 6-8 were established. The precursor 3 was also used to produce compound 9 by a first Mitsunobu reaction with adenine, followed by alcohol deprotection, similarly to the method previously adopted to obtain 5. The TBD-catalyzed substitution with i-propanol converted the methyl ester 9 to the analogue 10. Product 11 could be also obtained by reaction of 9 with a solution of methylamine. The inversion of configuration by the Mitsunobu reaction ascertained for 5 can also be assumed for 9, therefore 9 itself and its derivatives 10 and 11 have known absolute configurations.

Synthesis of Nucleosides 5-11
Seven nucleoside analogues were synthesized from the same common precursor 3, available by protecting the primary alcohol as tertbutyldimethylsilyloxy derivative of the methyl ester 2 accessible from the lactone LAC (1, Scheme 1). Structural modifications are introduced on the nucleobase moiety ( Figure 2) (as chloropurine in 5, 8, adenine in 9-11, methylpiperazinyl purine in 6, methylaminopurine in 7), and on the carboxyl-derived unit (methyl ester in 5-7 and 9, i-propyl ester in 8 and 10 and N-methylamide in 11).  In detail, compound 4 was obtained by a Mitsunobu reaction of 3 with 6-chloropurine in THF using diethyl azodicarboxylate (DEAD) and triphenylphosphine [10]. By treating 4 with tetrabutylamoniun fluoride, the compound 5 was produced. Based on Mitsunobu mechanism, a good leaving group is generated by reaction of the alcohol with triphenylphospine followed by a bimolecular nucleophilic substitution providing inversion of stereochemistry. 2D-NOESY experiment acquired for compound 5 in CDCl3 confirmed the derived (S) configuration at C-2, starting from the (R)-assigned in LAC. A correlation was observed between proton at 4.39 ppm assigned at C-4 position (of known (S) configuration based on LAC as starting chiral building block) with H-3 at 3.11 ppm, and for the other proton in position 3 at 2.76 ppm with the singlet at 8.40 ppm assigned to H-2′ in purine unit. Evidence from these experiments found support in the energy-minimized structure of 5 by density functional theory (DFT) calculation in chloroform, where the distances (2.307 Å and 2.033 Å, respectively) are compatible with the observed NOE ( Figure 4). The N-methylpiperazinyl moiety in analogue 6 and the N-methyl unit in derivative 7 were efficiently provided in good yields by treating the chloropurinyl compound 5 with the corresponding amines under microwave irradiation (MW) in methanol. Similar technique was adopted to obtain analogue 8 by triazabicyclodecene (TBD)-catalyzed transesterification of the methyl ester 5 with i-propanol. As derivatives from 5, the absolute configurations of the analogues 6-8 were established. The precursor 3 was also used to produce compound 9 by a first Mitsunobu reaction with adenine, followed by alcohol deprotection, similarly to the method previously adopted to obtain 5. The TBD-catalyzed substitution with i-propanol converted the methyl ester 9 to the analogue 10. Product 11 could be also obtained by reaction of 9 with a solution of methylamine. The inversion of configuration by the Mitsunobu reaction ascertained for 5 can also be assumed for 9, therefore 9 itself and its derivatives 10 and 11 have known absolute configurations.

Pharmacokinetics Studies
In this virtual screening, the synthetic compounds 5-11 were evaluated for their pharmacokinetic parameters, drug-likeness and ADME analysis, in comparison with remdesivir and its metabolite GS-441524. Besides respecting Lipinski's rule, the calcu-

Pharmacokinetics Studies
In this virtual screening, the synthetic compounds 5-11 were evaluated for their pharmacokinetic parameters, drug-likeness and ADME analysis, in comparison with remdesivir and its metabolite GS-441524. Besides respecting Lipinski's rule, the calculated pharmacokinetic parameters for all synthetic molecules (Table S1) are very promising, with a bioavailability score better than remdesivir (0.55 versus 0.17, Table S1). It is also pointed out in the bioavailability radar view ( Figure S8), obtained by Swiss ADME server which takes into account six physicochemical properties: lipophilicity, size, polarity, solubility, flexibility and saturation. A correlation of the molecular structures with these parameters (Table S1) highlights that (i) the lipophilicity is affected by the alkyl ester: the i-propyl unit increases the values as evident by comparison the corresponding analogues 5/8 and 9/10, as well as the presence of N-methylamide reduces the lipophilicity observed for the corresponding esters 9 and 10; the replacement of chlorine with an amino group is able to decrease the values in the order 5 > 7 ≈ 6 > 9 and 8 > 10, respectively; (ii) the polarity, expressed as topological polar surface area (TPSA) is in the range 99-128 Å 2 , to be compared with a value of 150 Å 2 for GS-441524 and of 213 Å 2 for remdesivir, and indicating a very favorable parameter for the compounds 5-11; (iii) the water solubility descriptors in the order 11 > 5-7, 9, 10, GS-441524 > 8 >> remdesivir, pointing out the potential of LAC-derived molecules as orally administered drugs. In addition, a high gastrointestinal (GI) absorption [11] for 5-10 (and low for 11, remdesivir and GS-441524) is predicted, as displayed in the white region of boiled-egg view ( Figure 5). Drug-likeness model score computed by MolSoft server considers a combined effect of physicochemical properties, pharmacokinetics and pharmacodynamics of a compound, given as a numerical value by comparison with 5000 market drugs (assigned positive value) and 10,000 molecules with non-drug properties (assigned negative values). In the evaluation of compounds 5-11, remdesivir and its metabolite, a wide variability was found, with the highest drug-likeness score (0.95) for compound 6 ( Figure S9). lated pharmacokinetic parameters for all synthetic molecules (Table S1) are very promising, with a bioavailability score better than remdesivir (0.55 versus 0.17, Table S1). It is also pointed out in the bioavailability radar view ( Figure S8), obtained by Swiss ADME server which takes into account six physicochemical properties: lipophilicity, size, polarity, solubility, flexibility and saturation. A correlation of the molecular structures with these parameters (Table S1) highlights that (i) the lipophilicity is affected by the alkyl ester: the i-propyl unit increases the values as evident by comparison the corresponding analogues 5/8 and 9/10, as well as the presence of N-methylamide reduces the lipophilicity observed for the corresponding esters 9 and 10; the replacement of chlorine with an amino group is able to decrease the values in the order 5 > 7 ≈ 6 > 9 and 8 > 10, respectively; (ii) the polarity, expressed as topological polar surface area (TPSA) is in the range 99-128 Å 2 , to be compared with a value of 150 Å 2 for GS-441524 and of 213 Å 2 for remdesivir, and indicating a very favorable parameter for the compounds 5-11; (iii) the water solubility descriptors in the order 11 > 5-7, 9, 10, GS-441524 > 8 >> remdesivir, pointing out the potential of LAC-derived molecules as orally administered drugs. In addition, a high gastrointestinal (GI) absorption [11] for 5-10 (and low for 11, remdesivir and GS-441524) is predicted, as displayed in the white region of boiled-egg view ( Figure  5). Drug-likeness model score computed by MolSoft server considers a combined effect of physicochemical properties, pharmacokinetics and pharmacodynamics of a compound, given as a numerical value by comparison with 5000 market drugs (assigned positive value) and 10,000 molecules with non-drug properties (assigned negative values). In the evaluation of compounds 5-11, remdesivir and its metabolite, a wide variability was found, with the highest drug-likeness score (0.95) for compound 6 ( Figure S9).

Docking Calculation and Molecular Dynamics
RdRp was selected as a drug target because this protein is essential for viral replication and transcription of SARS-CoV-2. Our computational approach took into account the potential inhibition by template-primer RNA covalently linked to remdesivir or the ligands 5-11 here investigated ( Figure 6). The process started from the nsp12-nsp7-nsp8 complex bound to the template-primer RNA and phosphate form of remdesivir structure deduced by cryogenic electron microscopy (cryo-EM) analysis at 2.5 Å of resolution (PDB ID: 7BV2). Protein and RNA-remdesivir were split, remdesivir was replaced by each

Docking Calculation and Molecular Dynamics
RdRp was selected as a drug target because this protein is essential for viral replication and transcription of SARS-CoV-2. Our computational approach took into account the potential inhibition by template-primer RNA covalently linked to remdesivir or the ligands 5-11 here investigated ( Figure 6). The process started from the nsp12-nsp7-nsp8 complex bound to the template-primer RNA and phosphate form of remdesivir structure deduced by cryogenic electron microscopy (cryo-EM) analysis at 2.5 Å of resolution (PDB ID: 7BV2). Protein and RNA-remdesivir were split, remdesivir was replaced by each monophosphate form of 5-11 and the obtained structure minimized by Yasara software. The free protein was optimized and docked with RNA by PatchDock server. The approach we have adopted in docking calculation is noteworthy, especially if related with similar systems reported in the literature. Recently, an in silico evaluation on the interaction of the full structure of remdesivir in the catalytic site of RdRp in the free form has been reported [12]. However, the mechanism of action was known for remdesivir acting as a prodrug to produce GS-441524 monophosphate, which is subsequently converted into the triphosphorylated form, able to be incorporated by the SARS-CoV-2 RdRp complex [8]. Furthermore, the RdRp considered (PDB ID 7BTF) contains no primer-RNA binding in the catalytic pocket, providing unreliable results. Conversely, we have applied a more rigorous method starting from a cryo-EM deduced structure [13] which contains template-primer RNA and monophosphate form of GS-441524 (PDB ID 7BV2) and available with a higher resolution in comparison with 7BTF. This establishes the active pocket for the binding of ligands. The presence of two interacting macromolecules resulted not easy treating them with a simple docking protocol. The approach included a first step where each single structure of template-primer RNA after replacement of GS-441524 with every molecule 5-11 was minimized by molecular mechanics, followed by application of a molecular docking algorithm based on shape complementary principles. monophosphate form of 5-11 and the obtained structure minimized by Yasara software. The free protein was optimized and docked with RNA by PatchDock server. The approach we have adopted in docking calculation is noteworthy, especially if related with similar systems reported in the literature. Recently, an in silico evaluation on the interaction of the full structure of remdesivir in the catalytic site of RdRp in the free form has been reported [12]. However, the mechanism of action was known for remdesivir acting as a prodrug to produce GS-441524 monophosphate, which is subsequently converted into the triphosphorylated form, able to be incorporated by the SARS-CoV-2 RdRp complex [8]. Furthermore, the RdRp considered (PDB ID 7BTF) contains no primer-RNA binding in the catalytic pocket, providing unreliable results. Conversely, we have applied a more rigorous method starting from a cryo-EM deduced structure [13] which contains template-primer RNA and monophosphate form of GS-441524 (PDB ID 7BV2) and available with a higher resolution in comparison with 7BTF. This establishes the active pocket for the binding of ligands. The presence of two interacting macromolecules resulted not easy treating them with a simple docking protocol. The approach included a first step where each single structure of template-primer RNA after replacement of GS-441524 with every molecule 5-11 was minimized by molecular mechanics, followed by application of a molecular docking algorithm based on shape complementary principles. This computational approach has provided the docking score and interface area values reported in Table 1. Higher values in both the geometric descriptor of shape complementarity [14] and approximate interface area reflect a better interaction in the complex. All molecules 5-11 showed higher values of both geometric shape score and interface area than GS-441524 (Table 1). As displayed in Figure 7, the results obtained by rigid docking calculation were in agreement with experimental data, validating the unconventional approach herein adopted for the particularly complex investigated system. This computational approach has provided the docking score and interface area values reported in Table 1. Higher values in both the geometric descriptor of shape complementarity [14] and approximate interface area reflect a better interaction in the complex. All molecules 5-11 showed higher values of both geometric shape score and interface area than GS-441524 (Table 1). Table 1. Geometric shape complementarity score and approximate interface area of the complex for each indicated compounds with free RdRp (7BV2) by computational analysis described in Figure 6. As displayed in Figure 7, the results obtained by rigid docking calculation were in agreement with experimental data, validating the unconventional approach herein adopted for the particularly complex investigated system.  Table 2 summarizes the specific interactions obtained for each ligand compound in the corresponding complex. The type and number of interactions involving compound 6 are better than the other LAC-derivative molecules, and even better than GS-441524, with the latter also displaying unfavorable interactions. These results are illustrated in Figure  8 for compound 6 compared with GS-441524, both linked as monophosphate, and in Figure S10 for the compounds 5 and 7-11. Table 2. Interactions (H-bond, π-π or π-alkyl, van der Waals, charge-charge or π-on and unfavorable) and in brackets the relative distance in Å evaluated by docking calculations of compounds 5-11 and remdesivir metabolite, each of them linked to RNA in the receptor pocket of free RdRp (7BV2).

GS-441524
U10 ( Table 2 summarizes the specific interactions obtained for each ligand compound in the corresponding complex. The type and number of interactions involving compound 6 are better than the other LAC-derivative molecules, and even better than GS-441524, with the latter also displaying unfavorable interactions. These results are illustrated in Figure 8 for compound 6 compared with GS-441524, both linked as monophosphate, and in Figure  S10 for the compounds 5 and 7-11. Table 2. Interactions (H-bond, π-π or π-alkyl, van der Waals, charge-charge or π-on and unfavorable) and in brackets the relative distance in Å evaluated by docking calculations of compounds 5-11 and remdesivir metabolite, each of them linked to RNA in the receptor pocket of free RdRp (7BV2).

Compound
H-Bond π-π π-Alkyl vdW ch-ch π-ion Unf.  Compound H-Bond π-π π-Alkyl vdW ch-ch π-ion Unf.   This type of study explicitly states which of the amino acids are involved and to compare them with the standard drugs [15], in this case remdesivir metabolite. The results by docking calculation for compound 6 linked to primer RNA interacting in the enzyme pocket show strong H-bonds with Ile 548 (Ile-C=O and MeNH + in N-methyl piperazinyl unit with a distance of 1.83 Å), Arg 553 (guanidine NH in Arg with purine N3 with a distance of 2.69 Å), Arg 555 (guanidine NH in Arg with OMe in ester unit, with a distance of 2.40 Å), Ser 759 (Ser-OH with O-tetrahydrofurane ring, with a distance of 2.41 Å) and Asp 760 (Asp-COOH Asp-NH both with O-phosphate with a distance of 1.18 Å and of 1.96 Å, respectively). Regarding the H-bonds involved for GS-441524 linked to primer RNA interacting in the pocket, Ser 759 (Ser-OH with 2 OH with a distance of 1.24 Å) is present similarly to compound 6. Asp760 (Asp-COOH with 3 O with a distance of 1.40 Å) is another common amino acid, but also involved in two repulsive unfavorable interactions with O-phosphate and 3-OH). In addition, Asn 691 (Asn-CONH 2 with 2 O, with a distance of 1.73 Å) and an unfavorable interaction with thiol unit in Cys 622. It is to note that the specific interactions involving the N-methyl piperazinyl unit with Ile 548 and the COOMe group for compound 6 are decisive for a better potential RdRp inhibition than for GS-441524.

GS
In light of score data on geometric shape complementarity and interface area (Table 1) and both the number of interactions and the corresponding distance (Table 2), 6 resulted as the most interesting molecule and therefore was selected for molecular dynamics (MD) simulation. In order to evaluate the stability in the time of RdRp free and in each complex with a sequence of RNA bonded to compound 6 or GS-441524, a 35 ns simulation has been performed on each structure. The potential energy of free enzyme and the complex with remdesivir metabolite resulted stable for all the simulation time, whereas for complex containing compound 6 showed to be stable after around 7 ns of simulation ( Figure S11).
Another parameter establishing the quality of structure is Z-score [Z = (x − µ)/σ], where x represents the energy of current structure, µ and σ are the average value and the standard deviation of energy in the "gold standard" protein population. For all structures, Z resulted about −1.35, meaning that each of them was satisfactory ( Figure S11). The radius of gyration (Rg), which is the mean-square mass-weighted root range of a set of atoms that shared the mass center [16] has been calculated. The Rg values for free RdRp and for both complexes showed similar values during all simulation time, very close to the one for the original X-ray protein, indicating a similar conformation flexibility ( Figure S12).
For evaluating the thermodynamic stability of the system, Root Mean Square Deviation (RMSD) was calculated, both for Cα and backbone as well as for all the heavy atoms. Similar values of the protein in the starting and in complexed structures were obtained, in the range of 1.2-1.9 Å ( Figure S12). In general, small values represent a minor flexibility in the structure [17,18].
Ligand movement RMSD after superposing on the receptor gives information on the movement of the ligand in its binding pocket. Average values 3.50 Å and 3.25 Å resulted for 6 and GS-441524, respectively (Figure 9). Regarding the fluctuation of amino acidic residues in the structure, Root Mean Square Fluctuation (RMSF) showed the involvement of the same amino acid residues, with very similar range of motion for both complexes under investigation ( Figure 10).

General
All reagents were purchased from Sigma Aldrich (WVR, Milan, Italy) and used without further purification. Preparation of amino acid 2 from LAC was carried out according to the reported procedure [3]. Microwave assisted reactions were carried out

General
All reagents were purchased from Sigma Aldrich (WVR, Milan, Italy) and used without further purification. Preparation of amino acid 2 from LAC was carried out according to the reported procedure [3]. Microwave assisted reactions were carried out using a Discover CEM microwave reactor. The reactions were not optimized and the yields were calculated for the products after chromatographic purification, based on the reacted starting material. Infrared spectra were recorded by using a FT-IR Tensor 27 Bruker spectrometer (Attenuated Transmitter Reflection, ATR configuration) at 1 cm −1 resolution in the absorption region 4000-600 cm -1 . A thin solid layer is obtained by evaporation of methanol solution of the sample. The instrument was purged with a constant dry air flux and clean ATR crystal as background was used. Spectra processing was made using Opus software package. NMR spectra were recorded on a Bruker-Avance 400 spectrometer using a 5-mm BBI probe 1 H at 400 MHz and 13 C at 100 MHz with values relative to TMS, in CDCl 3 (δ H 7.25 and δ C 77.00 ppm) or acetone-d 6 (δ H 2.05 and δ C 29 ppm) with δ values in ppm and J values in Hz; assignments were supported by polarization transfer (DEPT), heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) experiments; nuclear Overhauser enhancement (nOe) data from bidimensional NOESY experiments. Electron impact (EI)-MS and high-resolution HR-EI-MS spectra (m/z; rel.%) were recorded with a Kratos MS80 mass spectrometer equipped with home-built computerized acquisition software. Electrospray ionization (ESI)-MS mass spectra were recorded using a Bruker Esquire-LC spectrometer by direct infusion of a methanol solution (source temperature 300 • C, drying gas nitrogen, 4 L·min −1 , scan range m/z 100-1000). High-resolution ESI-MS measurements were obtained by direct infusion using an Orbitrap Fusion Tribrid mass spectrometer.
(3R,5S)-Methyl 3-hydroxy-5-(hydroxymethyl)tetrahydrofuran-3-carboxylate (2). The compound was obtained following the procedure reported and structural assignment is in agreement with known data [3]. In summary, the whole computational study provided data also better than remdesivir metabolite, all agree in indicating compound 6 as a promising candidate to deepen the study on a potential therapeutic inhibitor of SARS-CoV-2 RNA-dependent RNA polymerase. These findings emphasize the interest to deepen the study in next biological evaluation.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.